Add support for $getField aggregation operator.

See #4139
Original pull request: #4182.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ObjectOperators.java

@@ -17,6 +17,7 @@ package org.springframework.data.mongodb.core.aggregation;
 
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 
 import org.bson.Document;
 import org.springframework.util.Assert;
@@ -124,6 +125,16 @@ public class ObjectOperators {
 		public ObjectToArray toArray() {
 			return ObjectToArray.toArray(value);
 		}
+
+		/**
+		 * Creates new {@link GetField aggregation expression} that takes the associated value and obtains the value of the
+		 * field with matching name.
+		 *
+		 * @since 4.0
+		 */
+		public GetField getField(String fieldName) {
+			return GetField.getField(fieldName).from(value);
+		}
 	}
 
 	/**
@@ -283,4 +294,42 @@ public class ObjectOperators {
 			return "$objectToArray";
 		}
 	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $getField}.
+	 *
+	 * @author Christoph Strobl
+	 * @since 4.0
+	 */
+	public static class GetField extends AbstractAggregationExpression {
+
+		protected GetField(Object value) {
+			super(value);
+		}
+
+		public static GetField getField(String fieldName) {
+			return new GetField(Collections.singletonMap("field", fieldName));
+		}
+
+		public static GetField getField(Field field) {
+			return getField(field.getTarget());
+		}
+
+		public GetField from(String fieldRef) {
+			return from(Fields.field(fieldRef));
+		}
+
+		public GetField from(AggregationExpression expression) {
+			return from((Object) expression);
+		}
+
+		private GetField from(Object fieldRef) {
+			return new GetField(append("input", fieldRef));
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$getField";
+		}
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/spel/MethodReferenceNode.java

@@ -165,8 +165,8 @@ public class MethodReferenceNode extends ExpressionNode {
 				mapArgRef().forOperator("$dateSubtract").mappingParametersTo("startDate", "unit", "amount", "timezone"));
 		map.put("dateDiff", mapArgRef().forOperator("$dateDiff").mappingParametersTo("startDate", "endDate", "unit",
 				"timezone", "startOfWeek"));
-		map.put("dateTrunc",
-				mapArgRef().forOperator("$dateTrunc").mappingParametersTo("date", "unit", "binSize", "startOfWeek", "timezone"));
+		map.put("dateTrunc", mapArgRef().forOperator("$dateTrunc").mappingParametersTo("date", "unit", "binSize",
+				"startOfWeek", "timezone"));
 		map.put("dayOfYear", singleArgRef().forOperator("$dayOfYear"));
 		map.put("dayOfMonth", singleArgRef().forOperator("$dayOfMonth"));
 		map.put("dayOfWeek", singleArgRef().forOperator("$dayOfWeek"));
@@ -209,13 +209,6 @@ public class MethodReferenceNode extends ExpressionNode {
 		map.put("stdDevSamp", arrayArgRef().forOperator("$stdDevSamp"));
 		map.put("covariancePop", arrayArgRef().forOperator("$covariancePop"));
 		map.put("covarianceSamp", arrayArgRef().forOperator("$covarianceSamp"));
-
-		// TYPE OPERATORS
-		map.put("type", singleArgRef().forOperator("$type"));
-
-		// OBJECT OPERATORS
-		map.put("objectToArray", singleArgRef().forOperator("$objectToArray"));
-		map.put("mergeObjects", arrayArgRef().forOperator("$mergeObjects"));
 		map.put("bottom", mapArgRef().forOperator("$bottom") //
 				.mappingParametersTo("output", "sortBy"));
 		map.put("bottomN", mapArgRef().forOperator("$bottomN") //
@@ -233,6 +226,14 @@ public class MethodReferenceNode extends ExpressionNode {
 		map.put("minN", mapArgRef().forOperator("$minN") //
 				.mappingParametersTo("n", "input"));
 
+		// TYPE OPERATORS
+		map.put("type", singleArgRef().forOperator("$type"));
+
+		// OBJECT OPERATORS
+		map.put("objectToArray", singleArgRef().forOperator("$objectToArray"));
+		map.put("mergeObjects", arrayArgRef().forOperator("$mergeObjects"));
+		map.put("getField", mapArgRef().forOperator("$getField").mappingParametersTo("field", "input"));
+
 		// CONVERT OPERATORS
 		map.put("convert", mapArgRef().forOperator("$convert") //
 				.mappingParametersTo("input", "to", "onError", "onNull"));

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/ObjectOperatorsUnitTests.java

@@ -102,4 +102,11 @@ public class ObjectOperatorsUnitTests {
 				.isEqualTo(Document.parse("{ $objectToArray : " + EXPRESSION_STRING + " }"));
 	}
 
+	@Test // GH-4139
+	public void getField() {
+
+		assertThat(ObjectOperators.valueOf("batman").getField("robin").toDocument(Aggregation.DEFAULT_CONTEXT))
+				.isEqualTo(Document.parse("{ $getField : { field : \"robin\", input : \"$batman\" }}"));
+	}
+
 }

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/SpelExpressionTransformerUnitTests.java

@@ -1225,6 +1225,11 @@ public class SpelExpressionTransformerUnitTests {
 		assertThat(transform("dateTrunc(purchaseDate, \"week\", 2, \"monday\")")).isEqualTo("{ $dateTrunc : { date : \"$purchaseDate\", unit : \"week\", binSize : 2, startOfWeek : \"monday\" }}");
 	}
 
+	@Test // GH-4139
+	void shouldRenderGetField() {
+		assertThat(transform("getField(\"score\", source)")).isEqualTo("{ $getField : { field : \"score\", input : \"$source\" }}");
+	}
+
 	private Document transform(String expression, Object... params) {
 		return (Document) transformer.transform(expression, Aggregation.DEFAULT_CONTEXT, params);
 	}